1. Field of the Invention
The present invention relates to an image processing technique for suppressing image quality deterioration caused by halftone processing.
2. Description of the Related Art
In recent years, an image based on data processed by a personal computer (PC) is popularly printed on a printing material. Note that the number of tones of each pixel which can be expressed by a printing apparatus is normally smaller than the number of tones of each pixel which can be expressed by a PC. When image data is output to such printing apparatus, halftone processing for converting multi-tone image data into image data of a smaller number of tones is executed. As one halftone processing, an ordered dither method which decides an output value (quantized value) by periodically repeated comparison between a threshold and input value is known. As is known, the ordered dither method causes image quality deterioration corresponding to a difference between the threshold and a spatial frequency when an input image includes frequency components close to the spatial frequency corresponding to a period of the threshold. More specifically, jaggy of characters, disconnection of thin lines, interference fringes (moiré) with an input image, and the like are generated, and image quality deterioration which lowers an apparent resolution of an output image is caused.
In order to suppress such image quality deterioration, Japanese Patent Laid-Open No. 2011-155632 has proposed a technique for suppressing image quality deterioration by replacing a portion with a large error value by the ordered dither method in an image by data by another halftone processing method. On the other hand, Japanese Patent Laid-Open No. 2002-044449 has proposed a technique for deciding a weighting coefficient based on a size of an edge in image data, and deciding a pixel value to be output from pixel values of two halftone processes having different frequency components in accordance with the decided weighting coefficient. More specifically, as for a pixel having an edge larger than those of neighboring pixels, a weighting coefficient of the halftone process having a high frequency component is increased. Thus, a resolution of an edge portion which cannot be reproduced by the halftone process having a low frequency component is maintained.
Since the arrangement of Japanese Patent Laid-Open No. 2011-155632 performs correction using the periodic component of the threshold of the ordered dither method, when an image to be printed changes at a period shorter than that of the threshold used in the ordered dither method, its shape cannot be maintained. By contrast, the arrangement of Japanese Patent Laid-Open No. 2002-044449 performs correction of a deterioration portion while keeping a resolution of an image to be printed. However, in an electrophotographic image forming apparatus, a developing agent image of a certain pixel may often vary due to the influences of neighboring pixels, and the arrangement of Japanese Patent Laid-Open No. 2002-044449 cannot stably reproduce a shape of an edge portion.